Dichroic plate

ABSTRACT

A dichroic plate is disclosed for passing radiation within a particular frequency band and reflecting radiation outside of that frequency band. The value of the thickness of the plate is selected so that the plate acts as a resonant narrow band pass filter for the desired pass frequency, and the shapes of the apertures in the dichroic plate are selected to compensate for the phase shift caused by the air-plate interface presented to the signals passing therethrough.

1Z-Z-75 OR 399.24%239 United States Patent 1 1 1111 3,924,239 Fletcher et al. Dec. 2, 1975 [54] DICHROIC PLATE 3.231.892 [/1966 Matson ct al 343/909 3,252.160 5/1966 K2 343 786 [761 lnvemorsi James Fletcher, Admmlstmtor 3,633,206 1/1972 m c ia iimn 343/781 the National Aeronautics and Space il gi ggfi an Primary ExaminerEli Lieberman z g 2 I Attorney. Agent, or Firm-M0nte F. Mott; Wilfred a a1 Grifka; John 'R. Manning {22} Filed: June 27, 1974 [57] ABSTRACT [21] Appl. No.: 483,851

A dichroic plate is disclosed for passing radiation within a particular frequency band and reflecting radi- [52] US. Cl. 343/909 51 1m. 01. H01Q 15/04 outslde of that frequency band- The Value of the [58] Field of Search H 343/753, 754 755. 909, thickness of the plate is selected so that the plate acts 343/91] R as a resonant narrow band pass filter for the desired pass frequency, and the shapes of the apertures in the dichroic plate are selected to compensate for the [56] References Cited h h d b h f d UNITED STATES PATENTS p ase s 1ft cause y t e a1r-plate inter ace presente to the s1gnals passmg therethrough. 2,636,125 4/1953 Southworth 343/909 2.870.444 l/l959 Broussaud 343/909 6 Claims, 4 Drawing Figures US. Patent Dec. 2, 1975 3,924,239

DICHROIC PLATE ORIGIN OF INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

BACKGROUND OF THE INVENTION This invention relates to improvements in dichroic plates, particularly useful in dual frequency feed systems.

Dichroic plates are used in systems wherein; for example, it is desired to transmit or receive simultaneously X and S band signals along a common bore sight of a signal antenna, usually a Cassegrainian antenna. Usually, a dichroic plate is used-to reflect signals having a first frequency (such as the S band frequency) and pass signals therethrough of a second frequency (such as the X band frequency).

It is desirable for the dichroic plate reflector to attenuate, as little as possible, the signals that it passes therethrough, such as the X band signals, and that it not pass through any undesired signals. The dichroic plate apertures act as wave guides to the signals passing therethrough. Because of the free space to wave guide transition discontinuities at the (tilted) plate surfaces a differential electric and magnetic plane phase shift occurs which degrades the polarization of the signals and increases noise temperature. These conditions are most undesirable.

OBJECTS AND SUMMARY OF THE INVENTION An object of this invention is to provide a dichroic plate construction which minimizes the losses of the signals passing therethrough.

Another object of this invention is to provide a dichroic plate construction which minimizes polarization degradation and noise temperature contribution.

Still another object of this invention is to provide a dichroic plate construction which has sufficient physical thickness to insure mechanical stability under deleterious environmental conditions such as gravity, solar heating and wind.

Yet another object of this invention is the provision of a dichroic plate construction which is novel and useful.

These and other objects of the invention are achieved by determining the thickness of the dichroic plate as one which gives it the properties of a narrow band pass filter with respect to the frequency desired to be passed therethrough. Also, the apertures of the dichroic plate are shaped to substantially eliminate the phase shifts which occur to the signals desired to be transmitted therethrough which are caused by the free space to circular wave guide transition discontinuities resulting at the (tilted) plate surfaces.

The plate thickness is selected to approximately equal one-half the wavelength which the signals would have in a wave guide having the same configuration as the dichroic plate aperture. To eliminate the losses due to phase shift, the circular apertures have opposing flattened portions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front sectional view and FIG. 2 is a cross section of a dichroic plate reflector having a thickness in accordance with this invention.

FIG. 3 and FIG. 4 are respectively front and cross sectional views of a dichroic plate reflector having shaped apertures and a thickness in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a US. Pat. No. 2,636, l 25,to Southworth, there is described a dichroic plate reflector having either rectangular or circular apertures, and in which it is taught that a relatively thick disc or plate of metal, transversely apertured by punching closely spaced holes, may constitute an assemblage of wave guides that will readily permit transmission therethrough of electromagnetic waves of a frequency higher than the cutoff frequency while effectively attenuating and substantially precluding transmission of energy of frequencies lower than the cutoff frequencies. The design parameters for the thickness of the dichroic plate, in accordance with Southworth, in order to obtain his high pass filter results, are to make each aperture circular, having a diameter D and not transmitting therethrough electromagnetic waves of the wavelength not greater than approximately. 1.708D. For rectangular apertures, having a dimensiori'A, the waves which are transmitted therethrough should be below 2A. The thickness T of the plate which is the length of the individual wave guide channel should be preferably on the order of two wavelengths.

The present invention reduces the losses by selecting a thickness for the plate so that it acts as a resonant narrow band pass filter for the frequencies desired to be passed. This is achieved by making the thickness approximately equal to one-half of the wavelength that the pass band center frequency would have in a wave guide which has the same internal shape as that of an aperture through the dichroic plate. Thus, as shown in FIGS. 1 and 2, a dichroic plate 10 has a plurality of circular holes 12, therethrough. To improve dichroic plate performance, these holes are usually packed hexagonally. These holes in the dichroic plate effectively operate as juxtaposed circular wave guides. As shown in FIG. 2, which is a cross sectional view along the lines 22 in FIG. 1, the thickness of the wave guide 10, t equals approximately one-half the wavelength that the center frequency would have in a wave guide of the same shape as'the opening through the dichroic plate. As a result, the wave guides formed by the apertures in the dichroic plate operate to provide a very narrow band pass filter which is resonant at the frequency of transmittal. Thereby, there is a substantial improvement in the efficiency of transmittal. The thickness of a dichroic plate in accordance with this invention is approximately one-half the thickness of a Southworth plate for X band.

As a result of free space to circular wave guide transition discontinuity occurring at the (tilted) plate surfaces, a differential E- and H- plane phase shift occurs, which degrades the wave polarization and increases noise temperature. In accordance with this invention, in order to overcome these problems, a perturbation is placed within the circular wave guides formed by the holes through the dichroic plate, which introduces a compensating phase shift within the plate whichcounteracts that caused by the free space to circular wave guide transition discontinuities.

FIG. 3 shows a dichroic reflecting plate 14, in accordance with this invention. FIG. 4 is a cross section through the plate along the lines 44. The plate has a plurality of relatively hexagonally disposed apertures 16, except that each aperture has oppositely disposed flattened regions, (called flats), respectively 18, 20, opposite one another. If, considering a single column of holes, an incident wave upon the plate is considered as describing an incident plane, then the flats in the holes in a column are preferably placed with their centers in ,the incident plane. The noise temperature contribution and polarization degradation due to reflections at the dichroic plate surface are reduced as a result of the use of this aperture design. In an embodiment of this invention which was constructed with the use of the flattened areas, the plate ellipticity degradation was reduced from 1.8 dB to 0.4 dB, with the maximum noise temperature contribution reduced from 2.7 kelvins to 0.7 kelvins, over what is obtained using circular apertures.

The foregoing was accomplished in an embodiment of the invention which passed therethrough a center frequency of 8.415 GI-Iz, wherein the plate was 3.576 centimeter thick with an array of hexagonally packed 2.273 centimeter diameter holes drilled normal to the plate surfaces, with the center to center hole spacing being 2.388 centimeters. If the flat is considered as analogous to a chord which cuts off an arc of a circle formed by the hole, the distance from the outside end of the radius of the circle, which is perpendicular to the chord, in the embodiment of the invention was 0.043 centimeters.

In an article by J. R. Pyle entitled Cutoff Wavelengths of Wave Guides With Unusual Cross Sections, in the IEEE transactions on Microwave Theory and Techniques, Volume MTT-l2, No. 5, pages 556-557, September, 1964, correspondence, there is shown a wave guide with an opening of the type shown in FIG. 3 herein. It should be pointed out that Pyles letter described how to compute the cutoff wavelength of such a wave guide. In another letter by Pyle on page 557 there is described the design and performance of a circular polarizer comprising two metal plates set at 45 7 to the electric field vector E in a circular wave guide of radius r, carrying the TEll mode. Neither of these teach the use of this technique for the purpose of introducing a differential-E and -H plane phase shift to compensate for that caused by the air-wave guide interfaces. Further, it is now practical to numerically solve the wave equation in a cylindrical wave guide of arbitrary cross section, as given in the Pyle article, yielding highly accurate guide wavelength numbers as a function of the geometry. A computer program for this purpose was developed by Knud Pontoppidan as described in an article entitled Finite-Element Techniques Applied To Wave Guides Of Arbitrary Cross Sections, parts I and II" PHD Thesis, the Technical University of Denmark, Lyngby, Denmark, September, 1971. Thus, calculations required for the purpose of determining the specific shape and the thickness of a dichroic plate with apertures shaped in accordance with this invention can be solved using the Pontoppidan computer program. An embodiment of this invention having the dimensions previously given herein, which was designed for operation at a frequency of 8.415 GI-Iz, was constructed and tested. Optimum performance was observed at the designed frequency of 8.415 GHz and no pattern distortion, grating lobe response, or other unexpected behavior was observed.

By way of illustration, but not to be construed as a limitation on the invention, there follows a set of instructions whereby those skilled in this art may design the apertures for a dichroic plate, in accordance with this invention.

A. Select an angle ofincidence and frequency of operation at which the plate is to be electrically transparent B. Construct or design on a computer, (e.g., using the technique for thick perforated plate calculations described in an article by Chen in the IEE Transactions On Microwave Theory And Techniques, Volumn MTT 21, No. 1, January, 1973, page 1, entitled Transmission Of Microwave Through Perforated Flat Plates Of Finite Thickness), a dichroic plate with holes of circular cross section for best operation at conditions of (A).

1. The electric (E-) plane transparent frequency, designated fl and the magnetic (H-) plane transparent frequency, designated f should be approximately symmetrically disposed (one on each side of) about the desired transparent frequency f,,.

C. Accurately measure (and/or compute) fi and f,,,,.

D. For the circular cross section holes of (B) compute the guide wavelength, )tgh, atf and the guide wavelength, }tge, atj (standard computation). These are the required pyleguide cutoff wavelengths to be obtained at f,,, for the H- and E- planes, respectively.

E. Using standard formulas well known to those skilled in the art, compute, for the frequency 1",, a required E- plane guide cutoff wavelength, AcE, and a required [-1- plane guide cutoff wavelength, Xch, corresponding to Age and Agh, respectively.

F. Using Pyles FIG. 5 (P557) or (preferably) the exact Pontoppidan computer program, design the hole cross section (diameter and flat depth) to have E- and I-I- plane cutoff wavelengths of Ace and )tch, respectively.

G. The plate thickness and location of hole centers are not changed from the original circular cross section hole plate.

The heart of the design procedure is in steps (D) and (E), wherein the pyleguide, E- and I-I- plane properties at f are defined to be equal to the circular hole properties atf and f respectively. It should be noted that other cross sections can be used, such as round with fins but the pyleguide structure is easy to fabricate.

Where the incident wave at the transparent frequency of operation is to be a spherical wave rather than a plane wave, thus having varying angles of incidence on the dichroic plate from 0 to 60, a varying hole cross section (vs position on the plate) may be used. This is to be considered within the scope of the claims herein.

There has accordingly been described and shown a novel construction for a dichroic reflector plate whereby the efficiency of transmission signals there through, for the signals for which it was designed, are substantially increased. Noise temperature contributions and polarization degradation due to reflections at the dichroic plate surface are substantially minimized.

The embodiments of the invention in which an exclusive property or privelege is claimed are defined as follows:

l. A dichroic reflector plate of the type which reflects signals having a first frequency and passes signals therethrough having a second frequency, said reflector plate comprising a conductive plate having a pattern of holes therethrough, said conductive plate having a thickness which provides a resonant band pass response to signals including signals having said second frequency,

said conductive plate having a thickness which is substantially equal to one-half the wave length which said second signal would have within a wave guide which has the same shaped aperture therethrough as the shape of an aperture through said dichroic reflector plate.

2. A dichoric reflector plate as recited in claim 1 wherein the apertures therethrough are circular.

3. A dichroic reflector plate as recited in claim 1 wherein the apertures therethrough are circularly shaped except for two opposite parallel flat regions.

4. In a dichroic plate reflector, having apertures therethrough, the method of compensating for undesirable phase shift caused to signals passing through said apertures by the free space to aperture transition discontinuities, comprising shaping the cross section of each aperture which extends through said plate into a circle except for two opposite sides thereof which are flattened.

5. In a dichroic plate reflector, having apertures therethrough, the method of improving the efficiency of transmission therethrough of desired signals comprising establishing the thickness of said dichroic plate at a value equal to one half of the wavelength of said signals in a wave guide having the same cross section as the apertures in said dichroic plate.

6. In a dichroic plate reflector having apertures therethrough, the method of improving the efficiency of signal transmission and reducing the polarization degradation caused to signals passing therethrough comprising making said dichroic plate thickness equal to one half of the wavelength of said signals in a wave guide having the same cross section as an aperture in said dichroic plate, and

shaping the cross section of each aperture in said dichroic plate into a circle except for flat sections at opposite ends of one radius. 

1. A dichroic reflector plate of the type which reflects signals having a first frequency and passes signals therethrough having a second frequency, said reflector plate comprising a conductive plate having a pattern of holes therethrough, said conductive plate having a thickness which provides a resonant band pass response to signals including signals having said second frequency, said conductive plate having a thickness which is substantially equal to one-half the wave length which said second signal would have within a wave guide which has the same shaped aperture therethrough as the shape of an aperture through said dichroic reflector plate.
 2. A dichoric reflector plate as recited in claim 1 wherein the apertures therethrough are circular.
 3. A dichroic reflector plate as recited in claim 1 wherein the apertures therethrough are circularly shaped except for two opposite parallel flat regions.
 4. In a dichroic plate reflector, having apertures therethrough, the method of compensating for undesIrable phase shift caused to signals passing through said apertures by the free space to aperture transition discontinuities, comprising shaping the cross section of each aperture which extends through said plate into a circle except for two opposite sides thereof which are flattened.
 5. In a dichroic plate reflector, having apertures therethrough, the method of improving the efficiency of transmission therethrough of desired signals comprising establishing the thickness of said dichroic plate at a value equal to one half of the wavelength of said signals in a wave guide having the same cross section as the apertures in said dichroic plate.
 6. In a dichroic plate reflector having apertures therethrough, the method of improving the efficiency of signal transmission and reducing the polarization degradation caused to signals passing therethrough comprising making said dichroic plate thickness equal to one half of the wavelength of said signals in a wave guide having the same cross section as an aperture in said dichroic plate, and shaping the cross section of each aperture in said dichroic plate into a circle except for flat sections at opposite ends of one radius. 